It seems brown eyes are the majority eye color in the world. Why is this? Why are blue and green eyes less common?
Blue eyes are a recessive trait and are blue due to a lack of pigmentation so they are a genetic defect and can lead to retinal damage in excessively sunny environments. Green eye color is related to blue, if I remember right.
Brown is just the genetic default or norm.
It has to do with the way you inherit your eye color from your parents. You receive a gene for eye color from each of your parents. (This is a bit simplified; in reality there’s more than one gene involved.) The gene for brown eyes is dominant, which means that if you receive the gene for brown eyes from one of your parents, you will have brown eyes. The gene for blue eyes is recessive, which means you must receive the gene for blue eyes from both parents in order to have blue eyes. If you receive a gene for brown eyes from one parent and a gene for blue eyes from the other, you will have brown eyes, but your children may have blue eyes if their other parent also provides them with the gene for blue eyes. (Having two copies of the same gene is called being ‘homozygous’; having two different genes, one from each parent, is called being ‘heterozygous’. In case you don’t know, you have 22 paired chromosomes, one set from each parent, an X chromosome from your mother, and an X chromosome from your father if you are female or a Y chromosome if you are male. So you have two copies of each gene; usually only one copy will be active and the other will not be used in your body but might be passed on to your children.)
Eyes are naturally bluish, and all the other colors arise because of a pigment (brown eyes) or some other factor (tiny fat droplets, in green eyes). Green, hazel, gray and other eye colors are more complicated than blue and brown. In some populations, the genes for eye colors other than brown are rare or almost non-existent, and practically everyone has brown eyes. This probably means that the gene for blue eyes arose and spread in a population where it would be useful (northern Europe, for example, where there isn’t as much sunlight as in other places and making pigments to protect your eyes isn’t necessary).
There’s a technical discussion of eye color here and a simpler discussion here.
So if only one parent has blue eyes, and the other brown; and your eyes are blue, somethings up? What of those with one brown and one blue eye?
Seal tamer, that could be because the brown-eyed parent had a blue-eyed parent, and this blue-eyed grandparent’s recessive gene was inherited by the child along with the blue-eyed parent’s blue-eyed gene.
I’ve got a friend with one green and one hazel-brown eye - I think in these cases the physical expression of the eye-colour genes is complicated in some way. My friend has completely different optical prescriptions for each eye, as well.
Eye color genetics is complicated, but it’s worth pointing out that “recessive” and “rare” are not synonymous. Just because a trait is recessive doesn’t mean that it’s uncommon. You could easily have a population where 99% of the people display a recessive trait.
It’s the ther way around - two blue eyed parents can’t get a brown eyed child.
Because we’re the super power, silly.
My mother has blue eyes (bb) two recessive genes. My father has brown eyes. (Bb) one recessive and one dominant.
I got one b (recessive gene) from my mom and (b) one recessive from my dad.
My brown eyed brother got (b) recessive from my mom and (B) dominant from my dad.
I remember learning this in seventh grade science class and I thought it was SO cool. It’s a LOT more complicated than this, but this is the basic, easy to see stuff.
Or the person had a brown-eyed parent with one recessive blue gene. There’s the occasional blue-eyed Asian or African, even though the vast, vast majority of them are brown eyed. There’s a few recessive blue-eyed genes floating around that gene pool, but they’re not expressed until two brown eyed-parents who are both recessive blue carriers have children. Each of their children has a 1 in 4 chance of being blue eyed.
Remember that the actual genotypes are “brown” and “not-brown.” Blue is the most common “not-brown,” but hazel, green, yellow and purple are other “not-brown” eye colors. Which “not-brown” you have is determined by other, more complex genetics. In all the following examples, “blue” means “not-brown”, because it’s easier to speak that way.
I always found that little chart to be helpful in getting this.
B mean brown eyed gene.
b means blue eyed gene.
A person with BB (two brown genes) will have brown eyes.
A person with Bb (one brown, one blue gene) will have brown eyes, because brown dominantes over blue
A person with bb will have blue eyes, because there’s no brown to dominate over the blue
So if a brown eyed parent has a child, they have one of two possibilities (you can’t tell which they are), they’re either BB or Bb.
Let’s assume for the moment that Jane is BB.
Jane has a child with Mike, who is also BB.
Only one of each parent’s genes end up in an embryo. So in the little picture below, each square is a potential embryo.
Jane’s genes
M___B____B__|
i o|oooo|oooo|
k B|BB|BB|
e’o|oooo|oooo|
s B|BB|BB|
All the kids will have brown eyes, because there’s no blue genes in this mix.
Now, assume John has brown eyes, but has a hidden blue eye gene (Bb). If Jane and John have kids, their boxes look like this
Jane’s genes
J____B____B__|
oo|oooo|oooo|
h B|BB|BB|
n’o|oooo|oooo|
s b|Bb|Bb|
All of Jane and John’s possibilities will have brown eyes, but there’s a 50% chance they could have a kid who will later go on to parent a blue-eyed child, because John may pass on his recessive blue gene.
Cindy has brown eyes, but has a hidden blue gene (Bb), just like John. If they have kids, the boxes look like this:
Cindy’s genes
J____B____b__|
oo|oooo|oooo|
h B|BB|Bb|
n’o|oooo|oooo|
s b|Bb|bb|
Now Cindy and John have a 1 in 4 chance of having a blue-eyed baby (see it, there on the bottom right?) They also have a 50% (or 2 in 4) chance of having a brown-eyed child who may later present them with a blue-eyed grandbaby, due to those recessive blue genes.
Mack has blue eyes. There’s only one possibility: Mack has two recessive blue genes (bb). If he and Cindy have kids:
Cindy’s genes
M___B____b__|
ao|oooo|oooo|
c b|Bb|bb|
k’o|oooo|oooo|
s b|bb|bb|
Mack and Cindy have a 3 in 4 (75%) chance of having a blue-eyed baby. There’s only a 1 in 4 chance that their baby will be brown-eyed, and all their brown-eyed children are capable of parenting blue-eyed babies.
Finally, Nicole has blue eyes. There’s only one possibility: Nicole has two recessive blue genes (bb). If she and Mack have kids:
Nicole’s genes
M___b____b__|
ao|oooo|oooo|
c b|bb|bb|
k’o|oooo|oooo|
s b|bb|bb|
It’s all blue eyed babies, all the time.
So, if a brown eyed parent has a blue eyed child, in the absence of evidence of infidelity, we can assume they are Bb, that is, they have a recessive blue gene.
If a blue eyed parent has a brown eyed child, it’s time to call a lawyer. Someone’s been lyin’.
:smack: That should be, if TWO blue eyed parents have a brown eyed child, call a lawyer, of course. The one combo I forgot was if Jane and Mack have kids
Jane’s genes
M___B____B__|
a o|oooo|oooo|
c b|Bb|Bb|
k’o|oooo|oooo|
s b|Bb|Bb|
They’ll have kids with brown eyes, but every one of them will have the potential of parenting blue eyed babies, because they all inherit Mack’s recessive blue gene.
I think that’s your answer – it’s just how things started.
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But I remember hearing long ago that brown eyes are associated with quicker reflexes, something about hitting baseballs better, but I don’t know if I believe it. A quick, very cursory, 30 second internet search didn’t turn up anything reliable-looking.
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Also, this doesn’t answer the quesion, but it’s interesting:
http://www.straightdope.com/classics/a971205.html
I remember when we learnt the punnet square method of working out eye-colour in high-school, and I always wondered what the teacher would do if a student worked out that their daddy wasn’t really their daddy after Biology class one day… :smack:
I have no idea why I remember this because I hated that movie but during a biology class, the protagonist of Made in America figures out that her dad isn’t who her mother says it is because her blood type was incompatible with theirs.
The poor girl, who already suffered by having Whoopi Goldberg as her mother, eventually found out her dad was actually Ted Danson.
Spoiler for Made in America:
But by the end of the movie, she has learned that Ted Danson is NOT her father.
It always drives me nuts when movies and T.V. shows have someone discover that a baby doesn’t share the blood type of either the mother or the father, and therefore can’t be their child. Since my mother is AB, and my father is O, it actually is the other way around for my family. Any child who is either AB or O could not be their child (fortunately, we are all either A or B).